1. Field of the Invention
The present invention generally relates to medical devices that measure cardiac inter-beat intervals and analyze the cardiac inter-beat intervals. More particularly, the present invention relates to a method and apparatus for accurate detection of ectopic beats, robust removal of short and long cardiac inter-beat intervals that are related to ectopic beats, and construction of artifact-free cardiac inter-beat intervals.
2. Description of the Related Art
The variation of cardiac inter-beat (e.g., PP, RR) intervals results from both rhythmic activity of the heart electrical source and the dynamic properties of the cardiac conduction pathway, both of which are under autonomic control. In normal sinus rhythm, the RR intervals are known to fluctuate at various time scales, a phenomenon known as heart rate variability (HRV), which has been extensively investigated to probe the autonomic nervous activity. On the other hand, structural or functional abnormalities of the cardiac electrical conduction system can lead to cardiac arrhythmias.
The RR interval is a preferred choice to represent cardiac inter-beat interval due to easy acquisition of the electrocardiogram (ECG) signals, and the prominent QRS complexes present in these signals. The RR intervals not only can be easily measured from the surface ECG, but also can be measured from the subcutaneous ECG that is recorded by placing electrodes under the skin, or from the intracardiac electrogram (IEGM) that is recorded by inserting electrodes into the heart. Alternatively, the cardiac inter-beat intervals can also be obtained from other types of biosignals that are known to show the same rhythmic variation as the cardiac beats, including but not limited to, the blood pressure signal, the transthoracic impedance signal, the pulse oximeter signal, finger plethysmography signal, etc.
Abnormal cardiac intervals are usually evidenced by abrupt increase or decrease of the RR interval (or heart rate). One typical type of abnormal cardiac interval is caused by ectopic beat (EB) of either atrial or ventricular origin, characterized by abrupt shortening of the RR interval as compared to the preceding RR intervals. Another typical type of abnormal cardiac interval is the long pause after the Ectopic Beat. In fact, the short Ectopic Beat interval and the long post-Ectopic Beat pause often occur in tandem, characterized by a pair of short-long RR intervals in the RR interval tachogram. The short-long RR intervals can also repeat, resulting in the so-called bigeminy rhythm with alternating short and long RR intervals. Yet another type of abnormal cardiac interval is caused by consecutive Ectopic Beats, for example, the duplets, the triplets, or non-sustained ventricular tachycardia (NSVT), characterized by multiple consecutive short RR intervals in the tachogram. Yet another type of abnormal cardiac interval is caused by sudden drop of heart rate, for example, in patients with sick sinus syndrome or transient AV block, evidenced by abrupt increase of the RR intervals in the tachogram. Yet abnormal cardiac intervals can also be caused by transient sensing problems, for example, under-sensing of the R wave (resulting in abrupt increase of RR interval), over-sensing of the T wave (resulting in abrupt decrease of RR interval), or sensing of exogenous noise.
Detection of abnormal cardiac interval is a crucial step in time series analysis of RR intervals. For example, in Holter ECG analysis, daily Ectopic Beat counter (or Ectopic Beat frequency) is a simple yet important parameter for cardiac arrhythmia risk stratification. In another example, calculation of HRV parameters involves only normal cardiac inter-beat intervals, thus abnormal cardiac intervals must be removed prior to HRV evaluation. In addition, the heart rate turbulence (HRT) quantifies the short-term fluctuation in sinus cycle length that follows a ventricular Ectopic Beat, and has been shown to be a strong predictor of mortality and sudden cardiac death following myocardial infarction. Furthermore, the Ectopic Beat-free RR intervals can also be used to assess the baseline heart rate, its trend, and its circadian pattern.
Numerous techniques have been developed for automatic detection of Ectopic Beats from the ECG signals. One typical approach for Ectopic Beat detection is by means of morphological analysis of the ECG signals, based on the observation that ventricular Ectopic Beats typically have different QRS morphology than the normally conducted QRS morphology. However, this approach has several limitations. First, it cannot be used for Ectopic Beat detection from RR intervals only because it requires ECG morphological information. Second, depending on the source of the Ectopic Beats, the QRS morphology of the Ectopic Beats may not be necessarily different than that of the normally conducted QRS complexes. Third, morphology-based Ectopic Beat detection could not be applied to identify other types of abnormal cardiac cycles, for example, long ventricular pauses due to transient AV block or sudden drop of sinus rate.
Alternatively, Ectopic Beat detection can be achieved by analyzing time series of cardiac intervals. In implantable cardiac pacemakers and defibrillators, the Ectopic Beat detection is usually achieved by analyzing the atrial-ventricular relationship when sensing electrodes are placed in both atrium (e.g., RA) and ventricle (e.g., RV or LV). For example, a ventricular sense (VS) outside the ventricular refractory period is usually classified as a normal ventricular depolarization if it is preceded by an atrial event (atrial sense, or atrial pace) within a predefined time interval, or a ventricular Ectopic Beat otherwise.
In single chamber pacemakers or defibrillators, the Ectopic Beat detection becomes more challenging since atrial-ventricular association or dissociation could not be assessed. Conventionally, the ventricular Ectopic Beat detection is usually achieved by calculating the ventricular prematurity index by comparing each RR interval with the mean or median of previous several RR intervals. Similarly, the atrial Ectopic Beat detection can be achieved by calculating the atrial prematurity index by comparing each PP interval with the mean or median of previous several PP intervals.
Several other methods have been proposed for Ectopic Beat detection from time series of RR intervals. Most of these methods involve calculation of mean RR interval (or heart rate), standard deviation of RR intervals (or heart rate), and beat-to-beat difference of RR intervals (or heart rate). Other methods include polynomial fitting of the RR intervals, and median filter of the RR intervals. Based on our experience, none of these methods has satisfactory performance in terms of sensitivity and specificity of Ectopic Beat detection.
Morphological operators have been widely used in 2D image processing for noise removal, and have shown to have better edge preservation performance than other linear or nonlinear filters. The morphological operators have very high computation efficiency, and can be implemented in hardware platform, thus they are particularly suitable for application in low-power devices.
However, the application of morphological operators in 1D signal processing, in particular the ECG signal processing, has been limited. Morphological operators were used to implement a peak-valley extractor for QRS complex detection in ECG signals. Another morphological approach was developed to detect QRS complexes and remove baseline wander in neonatal ECG signals. Such approach was disclosed in U.S. Pat. No. 5,817,133 issued to Houben, for discriminating P waves from far-field R waves in an implantable pacemaker.